Many telecommunications systems employ switches which transmit radio frequency (RF) signals. One application for these types of RF switches is in spacecraft communications wherein an uplink RF signal may be transmitted as a downlink RF signal. These types of telecommunications systems frequently make use of large numbers of RF switches, with the individual switches controlled by a matrix switch driver arrangement. The matrix switch driver arrangement itself employs a matrix array of typically two-position switches to selectively activate individual RF switches by turning on the appropriate row and column switches. This approach is feasible because only one RF switch at any given time is activated in these types of telecommunications systems.
Referring to FIG. 1, there is shown a simplified combined block and schematic diagram of a prior art RF switch telemetry system 10. The RF switch telemetry system 10 includes the combination of an electromagnetic interference (EMI) filter 12 and a current limiter 14 connected to a power bus for energizing one or more RF switches which include coils L11, L21, L31 and L41. Each of these coils L11, L21, L31 and L41 is electromagnetically coupled to a respective magnetically sensitive arm (not shown for simplicity) for moving the arm in changing the position, or state, of the RF switch. Coils L21 and L41 are connected to the current limiter 14 by means of a first two-position switch S11, while coils L11 and L31 are connected to the current limiter by means of a second two-position switch S21. Diodes 26b and 26d are respectively coupled between coils L21 and L41 and the first switch S11. Similarly, diodes 26a and 26c are respectively coupled between coils L11 and L31 and the second switch S21. In the output stage, coils L11 and L21 are connected to a third two-position switch S31, while coils L31 and L41 are connected to a fourth two-position switch S41. A switch controller 15 is connected to each of the switches S11, S21, S31 and S41 for controlling the position of each of these four switches. By selectively actuating one of switches S11 and S21 and one of switches S31, and S41, any one of coils L11, L21, L31 and L41 may be actuated. As each of these four coils may be either in a single RF switch or may be in two or more RF switches, this arrangement allows either for four positions of one RF switch to be selected or for one or more positions to be selected in 2-4 RF switches under the control of a switch controller 15 connected to the four RF switch coils. Controller 15 may be either under automatic, pre-programmed control, or may be under the control of a communications system operator.
In many of these RF switch applications, the RF switches are remotely located, such as in an isolated telecommunications switching center or in a spacecraft. Because of the remoteness and unavailability of the RF switch network, it is essential for proper and reliable operation of the network that those responsible for operation of the telecommunications system know the status, or position, of each of the RF switches in the network. In the following discussion, the terms “position” and “state” of a switch are used interchangeably.
The position of the RF switch may be indicated by connecting a small microswitch 16 across the coils L5 of the RF switch is shown in FIG. 2. The microswitch 16 is ganged either physically or magnetically to a shaft of the RF switch which is electromagnetically coupled to coil L5, but is not shown in the figures for simplicity. This switch position indicating arrangement is provided by some RF switch manufacturers as an option. In the case of a spacecraft, separate wires must be run from the spacecraft telemetry unit(s) to each microswitch in order to interrogate the position of microswitch 16 and report it in the telemetry downlink. This approach suffers from the following disadvantages:                increases harness layout and a routing complexity, thus increasing the non-recurring harness cost;        adds significant harness weight and recurring cost;        increases spacecraft integration complexity and schedule; and        requires significantly more telemetry channel inputs in the telemetry subsystem hardware, increasing the weight and recurring cost of the hardware.        
The approach shown in FIG. 2 of connecting a microswitch 16 across the coil L5 of an RF switch for indicating the position of the switch is also impractical for two technical reasons. First, when the microswitch 16 moves from the open to the closed position, or vice versa, severe arcing will take place because the drive current to the coil 5 is on the order of 2 amps. This could result in damage to the microswitch 16, or even fusing of the contacts of the RF switch. In addition, when the microswitch 16 is in the closed position, it “steals” current from the switch coil L5 when actuated (all the drive current will flow through the microswitch and not the switch coil), thus making it impossible to change the position of the RF switch.
Simply adding a small resister 20 between an RF switch coil L6 and a microswitch 18 as shown in FIG. 3 for providing an indication of the position of the RF switch also does not solve the problem. While resister 20 may prevent damage to microswitch 18 when the microswitch moves from one position to another, this approach is also impractical. The current through microswitch 18 should be limited to no more than 10% of the total current required to activate the RF switch. The RF switch requires on the order of 2 amps through the approximately 15 ohm coil for roughly 500 milliseconds to ensure activation of the RF switch. Therefore, microswitch current should be limited to 0.2 amps, which results in a value for resister 20 of approximately 150 ohms. When microswitch 18 is open, the resistance of coil L6 is 15 ohms. However, when microswitch 18 is closed, the effective parallel combination of the 15 ohm coil L6 and the 150 ohm resister 20 is in the area of 13.6 ohms. The difference between 15 ohms and 15.6 ohms is difficult to determine even with a calibrated ohmmeter. However, the primary problem is that when “real world” component tolerances are applied (due to specification tolerances, temperature, aging, harness lengths, etc.), in the worst case it becomes virtually impossible to discriminate between open and closed microswitch states. Therefore, another, more reliable and repeatable approach is required for accurately and reliably monitoring the status of RF switches in a telecommunications system.
The present invention addresses the aforementioned limitations of the prior art by providing for the accurate and reliable control and monitoring of the position of one or more RF switches in a telecommunications system using existing RF switch wiring without increasing the weight, cost or integration complexity of the RF complexity of the RF switch position telemetry installation.